Case Study:

At the Kudankulam Nuclear Power Plant, engineers manage a nuclear reactor that generates electricity using controlled nuclear fission. The reactor primarily uses Uranium-235 as fuel. When a U-235 nucleus absorbs a slow neutron, it becomes unstable and splits into smaller nuclei, releasing a large amount of energy along with 2-3 neutrons.

This process creates a chain reaction, where released neutrons trigger further fission events. To maintain a stable reaction rate, control rods made of cadmium or boron are inserted into the reactor core. These rods absorb excess neutrons, preventing the reaction from becoming uncontrolled.

The energy released per fission is approximately 200 MeV. For a reactor generating 1000 MW power, about 3.1 × 10¹⁹ fissions occur per second. The heat produced is used to convert water into steam, which drives turbines to generate electricity.

The reactor also uses a moderator (like heavy water) to slow down neutrons and a coolant to remove excess heat. Safety systems are critical; if control rods fail, the reaction could escalate, leading to overheating.

In India, nuclear energy plays a key role in meeting rising electricity demands while reducing carbon emissions. However, concerns about radioactive waste disposal and safety remain important.

This case highlights the balance between energy efficiency, safety, and sustainability in nuclear power generation.

Questions:

Section A - MCQs

1. What is the primary function of control rods in a nuclear reactor?

A. Increase temperature
B. Absorb excess neutrons
C. Produce neutrons
D. Cool the reactor

2. Why is a moderator used in a reactor?

A. To increase neutron speed
B. To slow down neutrons
C. To absorb radiation
D. To generate electricity

3. What would happen if control rods are removed completely?

A. Reaction stops
B. Chain reaction becomes uncontrolled
C. Temperature decreases
D. No change occurs

4. What is the approximate energy released per fission of U-235?

A. 20 MeV
B. 200 MeV
C. 2000 MeV
D. 2 MeV

Section B - Short Answer Questions

1. Explain how a chain reaction is controlled in a nuclear reactor.

2. Why is a coolant necessary in nuclear reactors?

3. Calculate the total energy released per second if 3.1 × 10¹⁹ fissions occur per second and each fission releases 200 MeV. (1 eV = 1.6 × 10⁻¹⁹ J)

Section C - Long Answer Question

1. If the moderator is removed from the reactor, how will it affect the chain reaction and energy production? Analyze the consequences on reactor efficiency and safety.

Answer Key:

Section A - MCQs Answers:

1. B
2. B
3. B
4. B

Section B - Short Answers:

1. Control rods absorb excess neutrons, maintaining a steady rate of fission and preventing uncontrolled reactions.

2. Coolant removes excess heat from the reactor core, preventing overheating and ensuring safe operation.

3. Energy per fission = 200 MeV = 200 × 1.6 × 10⁻¹³ J = 3.2 × 10⁻¹¹ J

Total energy per second =
= 3.1 × 10¹⁹ × 3.2 × 10⁻¹¹
= 9.92 × 10⁸ J/s ≈ 992 MW

Section C - Long Answer:

• Without moderator, neutrons remain fast
• Reduced probability of fission --> chain reaction slows
• Energy production decreases significantly
• Reactor efficiency drops
• May lead to unstable or shutdown conditions
• Moderator is essential for sustained and controlled reactions